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MIT boffins say they're on the track of a nifty, portable machine which could turn dirty seawater into enough clean drinking water for several people - using no more power than a lightbulb.

Seawater desalination is already widely used, but conventional technologies such as reverse osmosis don't scale down well past a certain point. The new MIT ploy is the use of "ion concentration polarisation", delivered by arrays of minuscule fluidics devices manufactured in similar fashion to microchips - but using silicone rather than silicon.

According to MIT computing/electrical engineering brainboxes Sung Jae Kim and Jongyoon Han and colleagues in Korea, it should be possible to fabricate 1600 micro water purifiers on an 8-inch wafer. Such a unit would be able to purify 15 litres of water each hour, using "as much power as a conventional lightbulb" - say 60 watts, then.

The boffins envisage such kit being used in coastal disaster zones (for instance that of the Haiti earthquake recently) to provide drinking water at the point of use. A smallish generator and some ion-concentration kit would be able to supply a village, or a small household could survive on a pedalled or hand-cranked unit.

According to Han and Kim*, the kit isn't as efficient in terms of power as reverse osmosis, so it would only be for small-scale use. Even so, it could be handy in other situations than disaster relief. Small boats or yachts would be a possible market, or any coastal dwelling with access to electricity but not mains water. Small wind-up jobs could be a useful addition to survival kits and lifeboats.

A single unit has already been built and tested successfully using Massachusetts seawater purposely contaminated with "small plastic particles, protein and human blood".

"We clearly demonstrated that we can do it at the unit chip level," says Kim.

The next challenge will be the assembly of first a 100-unit system, then a field-scale 10,000-unit job able to supply a large group of people.

"We'll know if it's possible, and what problems might need to be worked on," according to Kim, in two years' time.

Fresh water is often assumed to be a finite resource in short supply, especially by green campaigners**. But this kind of tech shows that in fact for the vast majority of the human race that lives near the sea this is not the case. There's no such thing as a shortage of fresh water - just a shortage of power.

The engineers' paper, Direct seawater desalination by ion concentration polarization, is published in Nature Nanotechnologyhere. ®